With the progress in high precision machining technology, demands for diamond abrasives have been shifting to increasingly smaller particle sizes, to an extent that, in some cases, a surface roughness of 1 Å is required. The smallest diamond particles ever produced for abrasive applications are of “detonation” type, consisting of a mass of secondary particles that is an agglomeration of smaller, primary particles, which average 5 to 10 nm. It has been observed that this type of diamond, synthesized in a process where an explosive is combusted incompletely, has a lot of defects within the crystal and shows, when observed by transmission electron microscopy (TEM), rather round appearance commonly, as a result of the growth duration too short, being on the order of one-digit microseconds.
Individual diamond particles as yielded in a detonation technique are, as described above, very small and thus have a very active surface. They agglomerate readily to form secondary particles, firmly joined by means of non-diamond carbon or other substances that come in from the synthesis process and atmosphere. So this type of diamond behaves apparently as agglomerated particles with a size of 100 nm or more. It is also known that such secondary particles can be disintegrated into primary particles in a rigorous acid treatment.
Among the products of a technology based upon the extreme compression powered by chemical explosion or detonation, commonly known is the DuPont polycrystalline type diamond, which is produced by conversion from graphite under extremely high pressure by means of energy of chemical explosion. This type of diamond also is in a secondary particle structure: such that primary particles, commonly having a size of 20 to 30 nm, are fused in part and joined to each other under the extreme compression in excess of 30 GPa during the conversion process, trapping some graphite left unused. Consisting of firmly joined secondary particles of 100 nm or more, DuPont diamond also behaves as such; however this type, in contrast, cannot be disintegrated even by rigorous acid treatment. TEM microscopy shows that the primary particles do not exhibit idiomorphic faces but a somewhat spheroidal overall appearance, which is considered as an evidence of the limited conversion period.
Either of the above described processes is not adequate for the production of idiomorphic crystals, since they depend, for the compression of the starting material of low pressure phase of carbon, upon a chemical detonation, which, if extremely high in magnitude, lasts only for one-digit microseconds, a duration too short for the product to grow into such desired abrasive particles with sharp edges and points. So when used as an abrasive, the diamond products, which have few sharp edges or points, of the both techniques are short of achieving an efficient grinding rate, although the abrasive grits leave fine polishing marks in accordance with such small size of the primary particles.
On the other hand, static compression techniques can control properties such as shape, hardness and brittleness of the diamond product by operating at properly chosen pressure, temperature and time parameters to be applied. Further so produced diamond crystals can be readily crushed into very fine particles by impact-milling with steel balls.
It is observed under TEM microscopy that most of such fine particles, which have a size of tens of nanometers, are idiomorphic and have sharp edges, as a result of the crushing process that mostly takes place on the basis of the cleavage of diamond crystals. In some instances there are even flat triangular fragments of crystal observed with about 5 nm sides.
The Inventors have found that very fine powder or mass of very minute single crystalline diamond particles can be produced in a properly combined process of micro-crushing and precision grading. Based upon this finding we developed a technology for the production of 100 to 50 nm D50 diamond powder, for which we filed a patent application (published under Japan Kokai 2002-035636).
In the invention processed are particles of single crystalline diamond size-reduced by impact-crushing described above. The particles so crushed have commonly sharp edges and points to an extent that they often include some particles with a flat, regular triangular shape, as a result of well-known cleavage on (111) faces.
For crushing of the invention there are available such techniques as a handy process of ball mill with steel balls, while vibration mill and planetary mill can load more powerful impacts. A preferable crushing medium is steel balls for they have a sufficiently high density. Coarse diamond particles may also be used for the purpose of minimizing the contamination originating from the medium material.
Diamond particles as taken out from the crushing mill are first treated with chemical, in order to remove by dissolving debris of crushing medium having mixed during the process. The diamond particles then are subjected to a combined grading process of elutriation and centrifugation. In both processes the diamond particles are held in suspension and processed in the water, it is needed that the particles have affinity for water on the surface, in order to maintain stable suspension.
For this purpose a surface oxidization treatment is effective, whereby diamond particles are oxidized to attach on the surface such hydrophilic atom as oxygen or oxygen-containing group, as hydroxyl, carbonyl, and carboxyl, for example. For the surface oxidization, while heating to 300° C. or more in air may be available with a certain effect, a more reliable process may consist of a wet process, whereby diamond is treated in a bath comprising both one selected from sulfuric acid, nitric acid, perchloric acid, and hydrogen peroxide, and one selected from potassium permanganate, potassium nitrate and chrome oxide.
For preparing a good suspension of diamond particles in water it is necessary to minimize the overall concentration of ions that coexist in the water and, at the same time, to regulate the surface potential within the range adequate for establishing a good suspension. It is known that in weak alkaline condition diamond particles hold in suspension by repulsion each other of charges on the particle surfaces, so it is necessary to regulate the hydrogen-ion concentration and the zeta potential within the proper ranges, which are between pH 7.0 and 10.0 and between −40 and −60 mV, respectively.
While the elutriation technique is widely employed for the size grading of small diamond particles, it has a difficulty in that they need an extremely long precipitation time with such 100 nm or less diamond particles, resulting in a poor productivity. A combined process with super high speed centrifuge may somewhat increase the productivity; this approach is not necessarily realistic, because such equipment itself can be expensive, while there will be some problems in both maintenance and securing safety.